Short-time post-heat treatment method for improving welding seam performance of 6xxx series aluminum alloy

Abstract

The invention discloses a short-time post-heat treatment method for improving the performance of a 6xxx series aluminum alloy welding seam, and belongs to the technical field of aluminum alloy heat treatment. A welding workpiece is placed in a heating furnace, heated to 530-550 DEG C, subjected to heat preservation for 2-3 h and then subjected to water cooling to the room temperature; and then the welded workpiece is placed in a heating furnace to be heated to 160 DEG C to 190 DEG C, heat preservation is conducted for 2 h to 5 h, and then the welded workpiece is cooled to the room temperature. The method has the advantages that the strength and hardness of a welding seam of a welded workpiece can be remarkably improved, and good plasticity and excellent corrosion resistance are obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloy heat treatment, and particularly relates to a short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloys. Background Technique

[0002] 6xxx series aluminum alloys have good machining performance, high strength-to-weight ratio, good corrosion resistance, and weldability, and are widely used in the fields of aerospace, rail transit, and automotive manufacturing. Although using welding wires of similar grades usually results in better welding effects when welding aluminum alloy plates, there are few types of 6xxx series aluminum alloy welding wires, and they are difficult to prepare with complex processes. Due to their low melting point, good melt fluidity, good high-temperature stability, and low cost, 4xxx series aluminum alloy welding wires have been widely used in the welding of 6xxx series aluminum alloys.

[0003] However, using 4xxx series aluminum alloy welding wires to weld 6xxx series aluminum alloy parts usually results in low strength and poor corrosion resistance of the welds, restricting their application in various fields. In order to improve the performance of the welds, some scholars have proposed adding Mg and Cu alloying elements to the 4xxx series filler metal and performing post-weld aging heat treatment on the welded joints. However, the mechanical properties and corrosion resistance of the welds obtained by this method are still not ideal. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloys, which can improve the strength and hardness of the welds and obtain good plasticity and excellent corrosion resistance.

[0005] To solve the above technical problem, the technical solution of the present invention is a short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloys, including the following steps: S1. Solution treatment: Place the welded workpiece in a heating furnace, heat it to 530°C - 550°C, hold for 2 h - 3 h, and then cool it to room temperature by water cooling; S2. Aging treatment: Place the welded workpiece in a heating furnace, heat it to 160°C - 190°C, hold for 2 h - 5 h, and then cool it to room temperature.

[0006] Further, in step S1, it is heated to 540°C and the holding time is 2 h; in step S2, it is heated to 160°C and the holding time is 5 h.

[0007] Further, the base material of the welded workpiece is 6xxx series aluminum alloy after T6 heat treatment.

[0008] Further, the welding wire is a 4xxx series aluminum alloy added with 0.25 wt.%-0.4 wt.% Mg and 2.2 wt.%-2.3 wt.% Cu.

[0009] Further, the composition of the welding wire by mass percentage includes Zn: 0.9-1.1 wt.%, Mg: 0.25-0.4 wt.%, Cu: 2.2-2.3 wt.%, Mn: 0.001-0.002, Si: 8-10.5 wt.%, Cr: 0.001-0.002 wt.%, Ti: 0.001-0.002 wt.%, Fe: 0.01-0.15 wt.% and the balance Al.

[0010] Further, in the steps S1 and S2, the heating rate is 8 °C / min - 12 °C / min; the cooling rate is 40 °C / s - 100 °C / s.

[0011] Further, in the steps S1 and S2, the heating rate is 10 °C / min.

[0012] Further, the cooling method in the step S2 is water cooling or air cooling.

[0013] Further, the welding process adopted for the welded workpiece is cold metal transfer CMT welding.

[0014] Further, for the cold metal transfer CMT welding, the welding current is 110 A - 120 A, the welding voltage is 18.0 - 19.0 V, the wire feeding speed is 6.0 - 7.0 m / min, the welding speed is 6.5 mm / s - 7.5 mm / s, the shielding gas is high-purity argon, and the gas flow rate is 15 L / min - 20 L / min.

[0015] Advantages of the present invention: In the present invention, the weld obtained by welding a 6xxx series aluminum alloy after T6 treatment with a 4xxx series aluminum alloy welding wire containing Mg and Cu is first solution-treated to dissolve as much as possible the second phase in the weld into the α(Al) matrix to form a supersaturated solid solution, preparing for the subsequent precipitation of the second phase (spherical eutectic Si) during aging treatment; then aging treatment is carried out to precipitate a large amount of uniformly distributed second phase (spherical eutectic Si). While the strength and hardness of the weld are significantly improved, good plasticity and excellent corrosion resistance are obtained. Its mechanical properties are not inferior to those of the base material, and even in terms of tensile strength, it can exceed the base material (after measurement, the tensile strength of the base material before post-weld heat treatment is 356 MPa), and it has a wide range of application scenarios.

[0016] On the other hand, the heat treatment method in the present invention can enable the alloy to reach good mechanical properties within a short aging time, with a short production cycle and low processing cost. Description of the drawings

[0017] Figure 1 are the stress-strain curves of Examples 1-2 and Comparative Examples 1-3; Figure 2 are the polarization curves of Examples 1-2 and Comparative Example 1; Figure 3 are the micrographs of Examples 1-2 and Comparative Example 1 before corrosion; Figure 4 are the micrographs of Examples 1-2 and Comparative Example 1 after corrosion; Figure 5 are the micrographs of the microstructure of Comparative Example 1 after electrochemical corrosion test and the element distribution maps after local magnification; Figure 6 are the micrographs of the microstructure of Example 1 after electrochemical corrosion test and the element distribution maps after local magnification; Figure 7 are the micrographs of the microstructure of Example 2 after electrochemical corrosion test and the element distribution maps after local magnification. Specific Embodiments

[0018] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted here that the description of these embodiments is for helping to understand the present invention, but does not constitute a limitation to the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0019] Example 1 A short-time post-heat treatment method for improving the performance of 6xxx series aluminum alloy welds, comprising the following steps: S1. Solution treatment: Place the welded workpiece in a heating furnace, heat it to 530 °C, hold for 3 h, and then water-cool to room temperature; the heating rate is 8 °C / min; S2. Aging treatment: Place the welded workpiece in a heating furnace, heat it to 160 °C, hold for 5 h, and then cool to room temperature; the heating rate is 8 °C / min.

[0020] The welding process used is CMT welding, with a welding current of 110 A, a welding voltage of 18.1 V, a wire feeding speed of 6.4 m / min, a welding speed of 7.5 mm / s, a shielding gas of high-purity argon, and a gas flow rate of 20 L / min.

[0021] Example 2 A short-time post-heat treatment method for improving the performance of 6xxx series aluminum alloy welds, comprising the following steps: S1. Solution treatment: Place the welded workpiece in a heating furnace, heat it to 550 °C, hold for 2 h, and then water-cool to room temperature; the heating rate is 12 °C / min; S2. Aging treatment: Place the welded workpiece in a heating furnace, heat it to 190 °C, hold for 2 h, and then cool it to room temperature; the heating rate is 12 °C / min.

[0022] The welding process used is CMT welding, with a welding current of 120 A, a welding voltage of 18.7 V, a wire feeding speed of 7.0 m / min, a welding speed of 6.5 mm / s, the shielding gas is high-purity argon, and the gas flow rate is 15 L / min.

[0023] Comparative Example 1 No heat treatment is performed after welding. The welding process used is CMT welding, with a welding current of 110 A, a welding voltage of 18.1 V, a wire feeding speed of 6.4 m / min, a welding speed of 7.5 mm / s, the shielding gas is high-purity argon, and the gas flow rate is 20 L / min.

[0024] Comparative Example 2 A heat treatment method includes the following steps: S1. Aging treatment: Place the welded workpiece in a heating furnace, heat it to 180 °C, hold for 30 min, and then cool it to room temperature; the heating rate is 8 °C / min; S2. Repeat step S1 three times in total.

[0025] The welding process used is CMT welding, with a welding current of 110 A, a welding voltage of 18.1 V, a wire feeding speed of 6.4 m / min, a welding speed of 7.5 mm / s, the shielding gas is high-purity argon, and the gas flow rate is 20 L / min.

[0026] Comparative Example 3 A heat treatment method includes the following steps: S1. Solution treatment: Place the welded workpiece in a heating furnace, heat it to 540 °C, hold for 2 h, and then water-cool it to room temperature; the heating rate is 8 °C / min; S2. Aging treatment: Place the welded workpiece in a heating furnace, heat it to 200 °C, hold for 2 h, and then cool it to room temperature; the heating rate is 8 °C / min.

[0027] The welding process used is CMT welding, with a welding current of 110 A, a welding voltage of 18.1 V, a wire feeding speed of 6.4 m / min, a welding speed of 7.5 mm / s, the shielding gas is high-purity argon, and the gas flow rate is 20 L / min.

[0028] The base materials and welding wires of the welded workpieces used in the above-mentioned Example 1, Example 2, and Comparative Examples 1-3 are the same. The base material is 6082 aluminum alloy after T6 heat treatment. The composition of the welding wire includes, by mass percentage: Zn: 1.08%, Mg: 0.28%, Cu: 2.24%, Mn: 0.001%, Si: 10.44%, Cr: 0.0013%, Ti: 0.0013%, Fe: 0.01%, and the balance Al.

[0029] In order to verify the influence of the heat treatment process in this application on the specimens, hardness tests and mechanical tensile tests were respectively carried out on the welds of the samples prepared in Examples 1-2 and Comparative Examples 1-3. Corrosion resistance tests were also carried out on Examples 1-2 and Comparative Example 1.

[0030] I. Hardness test A Vickers hardness tester was used to conduct microhardness tests on each sample. The loading weight was 100 g, the loading time was 15 s. The fixed-distance multi-point test method was adopted, with an interval of 0.3 mm. A total of 8 points were tested, and the final result was the average value. The test results are shown in the following table: Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Hardness (HV) 99 107 78 90 118 It can be seen from the hardness test results in the above table that the hardness of the samples after the heat treatment of this application has all increased to a certain extent.

[0031] II. Tensile test A universal mechanical testing machine was used to conduct room-temperature tensile tests on each sample piece, and an extensometer was used to measure the distance. The tensile rate was 1 mm / min. The stress-strain curves obtained by tensile tests are as Figure 1 shown, and the yield strength, tensile strength, and elongation rate measured are shown in the following table: Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Yield Strength (MPa) 228 269 138 159 290 Tensile Strength (MPa) 342 365 274 277 361 Elongation (%) 8.43 7.33 7.90 5.12 3.92 It can be seen from the data in the above table that the hardness and tensile strength of the welds in Examples 1-2 and Comparative Example 3 have increased significantly, especially the increase in tensile strength is more obvious. Among them, the elongation rates of Example 1 and Example 2 have increased to a certain extent compared with Comparative Example 1 without heat treatment, indicating that they maintain good plasticity. For Comparative Example 3, although the strength has increased the most significantly, there is also an obvious decrease in the elongation rate, indicating that Comparative Example 3 has excellent strength but relatively general plasticity.

[0032] After detection, the composition of the weld includes Zn: 0.47%, Mg: 0.43%, Cu: 0.92%, Mn: 0.26%, Si: 4.73%, Cr: 0.047%, Ti: 0.019%, Fe: 0.12%, and the balance Al.

[0033] III. Corrosion resistance test The corrosion resistance was tested using an electrochemical workstation. The workstation adopted a three - electrode system, including a saturated calomel electrode as the reference electrode, a platinum foil as the counter electrode, and a working electrode made of the alloy under study. The exposed area of the polished surface of the sample was 0.25 cm 2 , and potential and anodic polarization measurements were carried out in a 3.5 wt% NaCl solution. The initial potential was - 1.4 V, the termination potential was 0 V, and the scanning rate was 2 mV / s. The polarization curves are as shown in Figure 2 . The polarization curves were fitted using CView software, and the fitting results are shown in the following table: Self-Corrosion Potential E0 Self-Corrosion Current Density i0 (A / cm2) Corrosion Rate (mm / A) Example 1 -0.76748 1.0406e-07 0.0012 Example 2 -0.73621 1.3373e-07 0.0016 Comparative Example 1 -1.04550 3.3855e-07 0.0040 When judging the corrosion resistance of materials based on the polarization curves, the more the curve is biased towards the upper left, the generally better the corrosion resistance. As can be seen from Figure 2 , the corrosion resistance of Example 1 and Example 2 has been improved to varying degrees compared with Comparative Example 1.

[0034] When judging the corrosion resistance of materials based on the fitting data, the self - corrosion potential E0, the self - corrosion current density i0, and the corrosion rate are all important indicators. The more positive the self - corrosion potential E0, the more corrosion - resistant; the more negative, the less corrosion - resistant. The smaller the self - corrosion current density i0, the better the corrosion resistance. From the polarization curve fitting data in the above table, it can be seen that the self - corrosion potential E0 of both Example 1 and Example 2 is significantly greater than that of Comparative Example 1; the self - corrosion current density i0 is significantly less than that of Comparative Example 1; and the corrosion rate is significantly less than that of Comparative Example 1. This shows that the heat treatment process claimed in this application can greatly improve the corrosion resistance of the specimens.

[0035] IV. Microstructure Observation Test Microstructure analysis was carried out using a scanning electron microscope. The microstructures of Example 1 - 2 and Comparative Example 1 before corrosion are as shown in Figure 3 . The microstructures, local magnified views, and element distribution maps of Example 1 - 2 and Comparative Example 1 after the electrochemical corrosion test are as shown in Figure 4 , 5 , 6, and 7 respectively.

[0036] As can be seen from Figure 3It can be seen that in Comparative Example 1, there are a small number of white irregular strip-shaped substances (indicated by red arrows) and light gray reticular tissue (marked by yellow frames) at the grain boundaries of the aluminum matrix. After EDS energy spectrum scanning and analysis, the white precipitate is Al2Cu, and the gray phase is eutectic Si. In the microstructures of Example 1 and Example 2, there are a large number of spherical structures and scattered white spots. After EDS energy spectrum scanning and analysis, the spherical structure is determined to be eutectic Si, and the white spots are Al2Cu. This is because during heat treatment, at the solution temperature, Si in the alloy tends to reduce the surface area to reduce the surface energy between Al and Si and lower the overall free energy of the alloy system, thereby forming stable spherical Si particles. It can be seen that after heat treatment according to the present application, the microstructural morphology and distribution of Example 1 and Example 2 have changed significantly.

[0037] It can be seen from Figure 4 that the number of corrosion pits in Comparative Example 1 is large, and they are closely connected and densely distributed. In Examples 1-2, there are a small number of obvious white spherical and shell-shaped corrosion products scattered. It can be seen that although local corrosion has occurred in both Comparative Example 1 and Examples 1-2, the corrosion degrees are obviously different, and the corrosion degree of Comparative Example 1 is much more serious than that of Examples 1 and 2.

[0038] It can be seen from Figure 5 that in Comparative Example 1, the Al matrix is severely corroded, and the Si element is distributed around the corrosion pits and at the grain boundaries. It can be seen from Figure 6 and 7 that in Examples 1-2, the spherical eutectic Si structure can still be seen scattered, and it remains after a long time of electrochemical corrosion. It can be seen that the presence of the spherical eutectic Si structure can hinder the expansion of corrosion pits or corrosion cracks, thereby improving the corrosion resistance. The surrounding of the white bright approximate spherical corrosion product is in a cracked state, and the surrounding fragmented flakes are also corrosion products. It can be seen from the element distribution map obtained by EDS energy spectrum scanning that the corrosion product is mainly formed by the enrichment of O element on the Al matrix to form an oxide, and an oxide film is formed on the surface, which can improve the corrosion resistance.

[0039] For Al-Si alloys, the Al matrix is the weak link of corrosion. In the as-welded state, the reticular eutectic Si divides the Al matrix into countless cellular structures. In a corrosive environment, Al is easily continuously corroded along the chambers, and multiple corrosion pits are easily connected and interact with each other. And due to the existence of a large number of Al-Si interfaces, the system free energy is increased and the corrosion potential is reduced. This is not conducive to the service of the product and is prone to fracture aging along the corrosion pits. After heat treatment according to the present application, the eutectic Si in the alloy is spheroidized, forming a spherical structure scattered in the matrix, disrupting the reticular structure and increasing the corrosion potential. This is more conducive to the uniform corrosion of the alloy, reduces the corrosion rate, and reduces the risk of corrosion cracking.

[0040] In summary, the analysis based on the corrosion morphology and corrosion products indicates that the heat treatment process claimed in this application can significantly improve the corrosion resistance of welded workpieces.

[0041] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions, and variations made to these embodiments still fall within the protection scope of the present invention.

Claims

1. A short-time post-heat treatment method for improving the performance of 6xxx series aluminum alloy welds, characterized in that: The steps include: S1. Solution treatment: Place the welded workpiece in a heating furnace, heat to 530℃-550℃, keep warm for 2 h-3 h, and then water cool to room temperature; S2. Aging treatment: Place the welded workpiece in a heating furnace, heat to 160℃-190℃, keep warm for 2 h-5 h, and then cool to room temperature.

2. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: In the step S1, the temperature is heated to 540° C. and the heat preservation time is 2 hours; in the step S2, the temperature is heated to 160° C. and the heat preservation time is 5 hours.

3. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: The base material of the welding workpiece is a 6xxx series aluminum alloy after T6 heat treatment.

4. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: The heating rate in steps S1 and S2 is 8°C / min-12°C / min.

5. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 4, characterized in that: The heating rate in steps S1 and S2 is 10° C. / min.

6. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: The cooling method in step S2 is water cooling or air cooling.

7. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: The welding process used for the welding workpiece is cold metal transfer (CMT) welding.

8. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 7, characterized in that: The cold metal transfer CMT welding has a welding current of 110 A-120 A, a welding voltage of 18.0-19.0 V, a wire feeding speed of 6.0-7.0 m / min, a welding speed of 6.5 mm / s-7.5 mm / s, a shielding gas of high-purity argon, and a gas flow rate of 15 L / min-20 L / min.

9. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 1, characterized in that: The welding wire is a 4xxx series aluminum alloy to which 0.25wt.%-0.4wt.% Mg and 2.2wt.%-2.3wt.% Cu are added.

10. The short-time post-heat treatment method for improving the weld performance of 6xxx series aluminum alloy according to claim 9, characterized in that: The composition of the welding wire includes, by mass percentage, Zn: 0.9-1.1 wt.%, Mg: 0.25-0.4 wt.%, Cu: 2.2-2.3 wt.%, Mn: 0.001-0.002, Si: 8-10.5 wt.%, Cr: 0.001-0.002 wt.%, Ti: 0.001-0.002 wt.%, Fe: 0.01-0.15 wt.% and the balance Al.

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